1. Field of the Invention
The present invention relates to an improvement of an outer ring and manufacturing method thereof for a shell-type radial needle bearing that is assembled, for example, in a rotation support section of a joint cross type universal joint called a Cardan joint.
2. Description of the Related Art
Conventionally, joint cross type universal joints called Cardan joints have been used, for example, in a steering apparatus or propeller shall of an automobile, for joining the end sections of a pair of rotating shafts that are not aligned with each other in order to be able to transmit a rotating force (torque). As illustrated in FIG. 6, this kind of joint cross type universal joint comprises: a pair of yokes 1a, 1b that are fastened to the end sections of both of the rotating shaft, the end sections of each yoke being fork shaped; and a joint cross 2. A pair of circular holes 3 that are concentric with each other are formed in the pair of tip end sections of each yokes 1a, 1b. The tip end sections of the shaft sections 4 of the joint cross 2 are supported inside these circular holes 3 by shell-type radial needle bearings 5 such that they can rotate.
As illustrated in FIG. 7, these shell-type radial needle bearings 5 comprise an outer ring 6 and a plurality of needles 7. The outer ring 6, for example, is made by performing a drawing process using a press on a metal plate as a raw material, and forming that raw material into a cylindrical shape with a bottom such that this outer ring 6 comprises: a cylindrical section 8 that fits inside the circular hole 3, a bottom plate section 9 that covers the opening on one end of the cylindrical section 8 (left end in FIG. 7), and a circular ring shaped inward facing flange section 10 that is formed by bending the other end of the cylindrical section 8 (right end in FIG. 7) toward the inside in the radial direction. The inner circumferential surface of the cylindrical section 8 functions as cylindrical concave shaped outer raceway 11. A spherical convex section 12 is formed in the center section of the inside surface of the bottom plate section 9. Moreover, needles 7 are provided between a cylindrical convex shaped inner raceway 13 that is formed around the outer circumferential surface on the tip end section of the shaft section 4 of the joint cross 2 and the outer raceway 11 such that the needles can roll freely.
In this state, the center section of surface on the tip end of the shaft section 4 comes in contact with the surface on the tip end of the curved convex section 12. In other words, in the example in the figure, by having only the center section of the tip end surface of the shaft section 4 come in contact with the tip end surface of the curved convex section 12 instead of having the entire tip end surface of the shaft section 4 come in contact with the inside surface of the bottom plate section 9, the friction force that acts on this area of contact is sufficiently reduced. As a result, together with preventing problems such as abnormal wear and burning of the area of contact during operation, the resistance to rotation in the universal joint is reduced by reducing the bending torque, which is torque necessary for causing the yokes 1a, 1b to swivel with respect to the joint cross 2. Moreover, by applying a suitable pressure in advance to the area of contact between the tip end surface of the shaft section 4 and the tip end section of the curved convex section 12, movement in the axial direction of the shaft section 4 in the inside of the shell-type radial needle bearing 5 is prevented, and thus movement inside joint section between the yokes 1a, 1b and the joint cross 2 is prevented.
Furthermore, in the example in the figure, by compressing and holding a circular ring shaped seal member 15 between a stepped section 14 that exists around the base end edge of the shaft section 4, and the inward facing flange section 10 of the outer ring 6, the internal space of the shell-type radial needle bearing 5 is sealed.
When both of the rotating shafts that are connected by way of the universal joint rotate, an axial load is repeatedly applied to the bottom plate section 9 of the outer ring 6 from the tip end surface of the shaft section 4. This axial load becomes larger proportional to the size of the rotation force that is transmitted by way of the universal joint. On the other hand, in the case of an automobile steering apparatus, which is one example of an apparatus that uses a universal joint, in recent years electrically powered power steering apparatuses are becoming popular. Of this kind of electrically-powered power steering apparatus, in a column type power steering apparatus in which an electric motor, which functions as the auxiliary power source, is located in a position (steering wheel side) further upstream in the direction of transmission of the rotation force during steering than the position of the universal joint, the rotation force that is transmitted by way of the universal joint becomes large. As a result, the axial load that is repeatedly applied to the bottom plate section 9 also becomes large, so unless the fatigue strength of the outer ring 6 is sufficiently maintained, there is a possibility that damage such as cracking will occur in the center section of the bottom plate section 9 or in the continuous section between this bottom plate section 9 and the cylindrical section 8 as illustrated in FIG. 8.
The fatigue strength of the outer ring 6 can be improved by increasing the overall thickness of the outer ring 6. However, by employing this method, the overall size and weight of the outer ring 6 is increased, and because of this, the size and weight of the universal joint also increases. In many cases there is also a need to make the universal joint more compact and lightweight. Therefore, in the field of universal joints that are assembled in the column-type electrically powered power steering, apparatuses above, being able to maintain durability without increasing the thickness of the outer ring 6 is desirable, while in other fields, being able to sufficiently maintain durability without reducing the thickness of the outer ring 6 is desirable.
Construction to meet these needs has been conventionally known, such as the construction illustrated in FIG. 9 wherein the cross-sectional shape of the bottom plate section 9a of the outer ring 6a is multi-stepped shaped (refer to JP03-62232(A), JP04-14819(U), and JP2006-125513(A)). By employing construction as illustrated in FIG. 9, it is possible to improve the fatigue strength of the bottom plate section 9a and the continuous section between this bottom plate section 9a and the cylindrical section 8 without having to increase the thickness of the outer ring 6a. However, by employing construction such as illustrated in FIG. 9, there are problems in that the design of the die used in manufacturing becomes more complicated due to the complexity of the shape of the bottom plate section 9a, as well as adjustment of this die becomes troublesome, productivity decreases, and there is an increase in manufacturing cost.
Furthermore, as described above, as the rotation force that is inputted to the universal joint increases, in the worst case, the shell-type radial needle bearing 5 that is pressure fitted into the circular hole 3 may come out from the yoke, and position shift in rotation of the rotating shaft may occur. In regards to this, surface processing such as surface texturing is performed on the outer surface of the outer ring 6 of the shell-type radial needle bearing 5. However, depending on the press processing, there is a problem in that performing this kind of surface processing may be difficult, so it may become necessary to perform a surface processing using a different process, which results in an increase in labor and a large increase in cost.
On the other hand, because the outer ring 6 is metal, there is also a need for the shell-type radial needle bearing 5 to be able to resist corrosion. Particularly, when the universal joint is used in an automobile, this shell-type radial needle bearing 5 is located in an environment that is exposed to wind and rain, so there is a special need for the outer ring 6 to have the ability to resist corrosion, so corrosion resistant coating may be applied to the outer ring 6. However, even in this kind of case, there is a problem in that sufficient adhesion of the anti-corrosive coating on the outer ring 6 that is normally used the shell-type radial needle bearing 5 may not be able to be obtained.
Furthermore, before assembly, there may be extremely minute oxides or extremely thin spot-like oxides already existing on the metal outer ring 6. Finding the existence of such oxides is very difficult, and when left as is, problems occur in that there is an increase in resistance when the pressure fitting of the bearing is implemented, and the oxide component may enter into the bearing during operation, so even the smallest oxides are removed beforehand. However, the work for removing these extremely small oxides, which are difficult to find, requires much labor and time, and increases cost.